Sputtered Tantalum Films Research Articles

A study of the phase transformation of low temperature deposited tantalum thin films using high power impulse magnetron sputtering and pulsed DC magnetron sputtering

In this study, tantalum thin films were deposited on silicon substrates using high power impulse magnetron sputtering (HiPIMS) and pulsed DC magnetron sputtering (pulsed DC), investigating the texture transformation behavior of α-Ta and β-Ta tantalum films by changing the duty cycle of tantalum targets from 5% to 88% at the power of 150 W and 50 W. The surface, microstructure, and texture of sputtered tantalum films were measured by scanning electron microscopy (SEM), atomic force microscope (AFM) and X-ray diffraction spectroscopy (XRD). The hardness and resistivity were measured by nanoindentation and a four-point probe. The results reveal that the impulse power and voltage of the Ta target were increased because of the lower duty cycle. The higher the ion energy, resulting in a more efficient atomic mobility, the texture phase of tantalum film gradually changes from β-Ta-dominated to α-Ta-dominated, hardness and resistivity shift from β-Ta to the α-Ta property with the decreasing duty cycle. By adjusting the duty cycle and controlling the power, the texture transformation of tantalum film occurs. Different from the previously complicated transformation process, the tantalum thin film structure with high density and high quality deposited using this approach provides a broader range of applications.

On the origin of the metastable β-Ta phase stabilization in tantalum sputtered thin films

Despite extensive studies since the 1960s, the exact mechanisms underlying phase formation of metastable tetragonal beta-Tantalum (β-Ta) structure commonly found in magnetron sputtered tantalum films have not yet been unraveled. Here, we combine in situ and real-time multiple beam optical stress sensor (MOSS) during sputter-deposition of Ta films together with ex situ X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) to gain insights into the early growth stages and interface properties. The nucleation conditions were varied by depositing films on different buffer layers, either amorphous (a-Si, a-SiOx, a-SiNx) or crystalline (c-Mo), as well as changing the substrate temperature (Ts = 300–750 K). The influence of the mean deposited energy, in the range of 25–83 eV/atom, as calculated from Monte Carlo simulations, on the resulting stress state and phase formation was also investigated by varying the Ar working pressure (0.12–0.75 Pa) and substrate bias voltage (0 to −190 V). It is shown that the nucleation step is decisive for the stabilization of β-Ta phase on amorphous layers. After the initial formation of an interfacial amorphous Ta(Si) alloyed layer, β-Ta nuclei form due to interface energy minimization and grow columnar with a strong [001] fiber-texture. No transition to the equilibrium bcc alpha-phase (α-Ta) was found with increasing film thickness or variation of the deposited energy. The results are interpreted based on a thermodynamic description of nucleation and growth theory. Such rationale also qualitatively supports experimental findings of α-Ta phase selection with the assistance of substrate heating or by by-passing the nucleation stage upon direct growth in registry on crystalline Mo (110) seed layers.

Unstable kinetic roughening during the island coalescence stage of sputtered tantalum films

Kinetic roughening of tantalum films during the initial growth stages has been studied by atomic force microscopy, scanning electron microscopy, and dynamic scaling theory. Different from the time-independent scaling behavior for continuous film growth, an intriguing unstable kinetic roughening occurs during island coalescence. In such case, roughness exponent α increases with growth time, accompanied by lower growth exponent β and higher coarsening exponent η. Detailed analysis of film surface morphology and simple phenomenological models suggests that this unstable behavior is related to the pronounced lateral growth of surface islands, which arises from the combined effect of the formation of grain boundary and the covering of heterogeneous substrate surface.

Real-Time XRD Characterization of Growth of Sputtered Tantalum Films

ABSTRACTAn in-situ DC magnetron sputter deposition system was constructed on the goniometer of an X-ray diffractometer for investigation of thin film nucleation and growth in real time. Tantalum films were deposited on silicon and A723 steel substrates under various deposition conditions. This paper showed: 1) Ta films evolved in single or mixed α-Ta and β-Ta phase on Si and Fe with very different growth characteristics. 2) Crystalline phase was sensitive to deposition mechanics, Ta film changed phase spontaneously without external parameter changes. 3) In pressure range 0.65–13 Pa at 100 watts, phase was not sensitive to pressure variations, but deposition rate and degree of texture decreased at high gas pressure. 4) Depositions using heavier gas showed higher a-Ta concentrations. 5) Reverse sputter cleaning of target and substrate surfaces improved Ta film adhesion.

Phase formation and microstructure changes in tantalum thin films induced by bias sputtering

The effects of argon-ion bombardment on the structure and properties of sputtered tantalum films have been studied. Applied substrate bias voltage was used to control the bombardment energy in a hollow-cathode-enhanced low-pressure magnetron sputtering system. The films were characterized by x-ray diffraction, electrical measurements, Rutherford backscattering spectrometry, and stress measurements. The findings concerning the effects of negative substrate bias on film resistivity and structure run counter to earlier work. In particular, as opposed to results found in many early studies, which primarily involved higher-pressure discharges, at zero bias voltage the films have low resistivity and contain the bcc phase. Increasing the bias to −100 V, increases the resistivity dramatically, and induces formation of β-Ta, with no significant change in film impurity levels. The difference from earlier work is attributed to the lower relative impurity flux, as well as more energetic substrate bombardment in low-pressure magnetron sputtering. Energetic substrate bombardment is clearly demonstrated by the high level of argon content in films deposited with no applied bias (2%). At very high bias voltage, argon incorporation increases dramatically and resistivity increases in the β phase. Additionally, a decrease in the compressive stress and change in preferred orientation occur. The results suggest that the formation of the β phase is not controlled by impurity effects, but by Ta forward scattering and related stress changes.

Residual Stress Effects in the Scratch Adhesion Testing of Tantalum Thin Films

ABSTRACTThe effect of residual stress on the scratch adhesion critical load has been measured for sputtered tantalum films. In this single metallurgical system, six different failure modes could be observed, ranging from ductile ploughing to extensive spallation. For tantalum films deposited on silicon substrates, a 15% decrease in critical load was observed as the film residual stress increased from -1.1 GPa to +1.0 GPa. A larger percentage decrease (50%) was observed for films deposited on softer AIMg/NiP substrates. Film spallation was more extensive for films deposited over a thin carbon layer and for these films critical loads increased slightly with film stress. These results are substantially in contradiction with existing quantitative models for the scratch adhesion test and indicate that failure by shear at the film-substrate interface can be more important than failure by compressive buckling.

Effects of oxidation conditions on the properties of tantalum oxide films on silicon substrates

Tantalum oxides have been prepared by thermal oxidation of sputtered tantalum films on silicon and silicon dioxide substrates at temperature between 450°C and 700°C. The composition of such oxide is tantalum pentoxide (Ta 2O 5) and the amount of silicon diffused into the tantalum oxide from the silicon substrate increases as the oxidation temperature increases. The temperature dependence of the dielectric constant of Ta 2O 5 on silicon can be explained in terms of its silicon concentration. CV curves of Al/Ta 2O 5/Si capacitors reveal the presence of donor-type interface states whose density decreases as the oxidation temperature increases. The flat band voltage of Al/Ta 2O 5/SiO 2/Si capacitors become more negative as the oxidation or annealing temperature increases regardless of the annealing ambient.

Tantalum Film Capacitors with Improved AC Properties

Tantalum thin-film capacitors have been fabricated from magnetron sputtered tantalum films with different nitrogen concentrations. Capacitors made from film containing between 13 and 22 atom percent nitrogen and heat treated at temperatures between 250°C and 350°C show lower ac losses and TCC, and improved capacitance stability, when compared to standard capacitors based on tantalum containing 3 atom percent nitrogen. In addition, the dc properties of the improved capacitors are equivalent or better than those of the standard product. It is proposed that in capacitors containing over 13 atom percent nitrogen, the nitrogen fills interstitial vacancies in the tantalum lattice. A barrier is therefore produced to substantial oxygen diffusion from the dielectric into the underlying tantalum film during exposure to temperature between 250° and 350°C. In addition, results at 200°C suggest that a second mechanism for improved characteristics is the removal of water or citrate molecules, incorporated during anodization, from the hulk of the dielectric.

Thin-film hybrid circuits

Thin-film hybrid circuits are of prime interest due to their improved temperature stability and reduced parasitics. Sputtered tantalum films and vacuum-evaporatod aluminium films are used to fabricate thin-film resistors and capacitors on passive substrates. Some of their physical properties are investigated in order to use them in electronic circuits. As an illustrative application of these RC networks, the following circuits are built using discrete transistors: thin-film ladder networks for digital-to-analogue converters, resistor-transistor logic circuits, a Schmitt trigger circuit and an astable multivibrator circuit. The design procedure, fabrication details and result of testing are discussed at length for ladder networks which are of special interest in digital communication. The paper concludes by giving details of our proposed activities in this field.

The Fourier analysis of the x-ray diffraction profiles obtained from sputtered tantalum films

A method using Fourier analysis of the X-ray diffraction profile is proposed for estimating the nitrogen content in sputtered tantalum films. The results obtained from computer simulation of a two-crystalline system consisting of b.c.c. tantalum and tantalum nitride revealed that the third harmonics of the Fourier components (derived from the diffraction profile of the Cu Kα characteristic ray for 2 θ = 30−45°) could possibly be used as a measure of the nitrogen content of the films. The experimental results confirmed this, and the temperature coefficient of resistance of the films plotted as a function of the third Fourier component showed a plateau region for a Fourier component from −0.1 to +0.6.

Birefringent waveguides in thermally oxidized nitrogen-doped sputtered tantalum films

Nitrogen-doped tantalum films were deposited by d.c. sputtering in a semicontinuous d.c. diode system. Their electrical properties varied with nitrogen in a similar manner to that generally reported; e.g. the resistivity first decreased from 200 μω cm to a minimum value of 140 μω cm and then increased monotonically. The film density also decreased with increasing nitrogen flow. The films were oxidized both by anodization and by the thermal oxidation in an oxygen atmosphere at 550°C. For a given anodization voltage, the capacitance decreased by approximately 50% when the nitrogen content in the sputtered film reached an estimated 30 at.%. The thermal oxides acted as waveguides for He-Ne light and their refractive indices were strongly dependent on nitrogen concentration. For Ta 2O 5, the indices for TE and TM modes were 2.21 but both decreased by different amounts as the nitrogen concentration increased. For the highest nitrogen concentration, the values for the TE and TM modes were 1.91 and 1.82 respectively. The birefringence must be due to local asymmetry around the Ta atom which is caused by the presence of nitrogen but the mechanism responsible for the ordering in amorphous or randomly oriented micropolycrystalline films is not clear.

Dielectric properties of films prepared by the reactive sputtering of Ta In [formula omitted] mixtures

The dielectric properties of amorphous films prepared by reactively sputtering tantalum in oxygen-nitrogen mixtures have been measured. As the nitrogen content in the sputtering discharge was increased, the dielectric constant decreased from 26 to 16 and the films became more suitable for capacitor fabrication ( i.e. had lower leakage currents and higher breakdown voltages). Comparison with reported results for anodic films indicates that the nitrogen concentration in the films reached approximately 7 at.% at the lowest dielectric constant. Sputtered tantalum films were found to be suitable for base electrodes, whereas aluminum electrodes significantly increased the capacitor leakage currents.

Changement de structure des couches de tantale pulvérisées au cours de la stabilisation des conditions de dépôt

The structure of sputtered tantalum films deposited by d.c. biased d.c. diode sputtering has been studied. β tantalum is observed under equilibrium deposition conditions, when consistent and reproducible results are obtained. The structure of the films sputtered during the period of stabilizing deposition develops from the b.c.c. phase to the β phase through mixtures of the two. Based on the assumption that β tantalum is impurity stabilised, it is concluded that the source of impurities in our system is the thick film of tantalum grown on the discharge electrodes, collector and anode.

Quantitative analysis of light elements (nitrogen, carbon, and oxygen) in sputtered tantalum films by Auger electron spectroscopy and secondary ion mass spectrometry (SIMS)

The quantitative analysis of light elements (N, C, O) in sputtered tantalum films by Auger electron spectroscopy and secondary ion mass spectrometry (SIMS) via calibration with standards and electron microprobe analysis is described. The calibration standards were prepared by reactive sputtering, and the homogeneous distribution of the dopants (N, C, O) in these standards was established by SIMS and Auger in-depth profiling measurements. Although the electron microprobe could reproducibly detect the presence of low levels (< 5 at.%) of N, C, and O, the accuracy of quantitative electron microprobe measurements for these light elements in tantalum below ~5 at.% was questionable due to the lack of sufficiently accurate x-ray absorption coefficients. The accuracy of electron microprobe quantitative results above ~5 at.%, however, was estimated to be within plus or minus 10%. The analysis of nitrogen, carbon, and oxygen below ~5 at.% was accomplished by an extrapolation of the normalized Auger and SIMS data. SIMS detection limits for oxygen and carbon were in the ppM range and for nitrogen in the 0.1 at.% range depending on instrument background and amount of sample consumed. The Auger detection limits for N, C, and O were in the 0.3-0.4 at.% range.

Losses in tantalum pentoxide waveguides

Thin films of tantalum pentoxide for possible use as waveguides have been prepared by reactive sputtering and by thermal and anodic oxidation of sputtered tantalum films. Measurements have been made of the losses in these films for different guided modes. These results have been analyzed to determine the loss mechanism. The losses do not fit the Rayleigh scattering theory precisely and the deviation is attributed to absorption by impurities at the surface as well as in the bulk. The effects of bulk and surface loss mechanisms have been obtained and causes of these are discussed.

The Effect of The Light Elements Nitrogen, Carbon and Oxygen on The Physical Properties of Sputtered Tantalum Films

The structural and electrical properties of reactively sputtered tantalum films are shown to be a strong function of the nitrogen, carbon and oxygen concentration in the films and of sputtering voltage. Transitions from the tetragonal β-Ta structure to a mixture of β-Ta and bcc-Ta, and finally to a single phase bcc-Ta region, followed by nitride, carbide or oxide phases have been observed by X-ray diffraction. In addition, film structure has been correlated for two sputter conditions to Ta-film composition as determined by Auger electron spectroscopy combined with ion sputtering during analysis. It will be shown that once the functional relationships between sputtering voltage, composition and film properties have been established, the monitoring of resistivity, TCR, and spectrophotometry of 60 V anodic films can serve a diagnostic control function in the deposition of the Ta-film component materials.

Electron microprobe analysis of the Ta-O system

The detection system of an electron microprobe was modified to increase its sensitivity to soft x-rays, absorbed oxygen in a tantalum single crystal being readily detected. The oxygen concentration within about 400 Å of the surface was determined for a number of samples by comparison with a set of standards. The standards were sputtered tantalum films with oxygen concentrations between 8·3 and 71·4 at%, as determined from anodization efficiencies. For the single crystal the oxygen concentration was approximately 8 at%, but ion-scattering spectrometry showed that the oxygen concentration decreases with increasing distance from the surface. The energy of the O Kα radiation was 6·0 ± 1·4 eV higher than for Ta2O5, and the oxygen may be present interstitially in the tantalum crystal. Similar results were obtained for a sputtered tantalum film with the BCC structure. For a sputtered β-Ta film the oxygen concentration was approximately 10 at% and did not decrease with increasing depth. The oxygen is probably incorporated during deposition and the O Kα energy was the same as for Ta2O5. Tantalum foil annealed in oxygen at temperatures up to 600 °C increased in oxygen content according to an activation energy of 0·47 ± 0·08 eV. At 350 °C, the surface concentration was 30 at%, but no second phase was observed. Segregation of a second phase, with an oxygen concentration of 58 at%, was observed at 400 °C, and Ta2O5 was formed at higher temperatures.

The effects of argon pressure on the properties of sputtered tantalum films

The properties of tantalum films sputtered onto glass substrates over a wide range of sputtering conditions have been investigated. The films were sputtered at argon pressures between 9 and 80 mtorr and at voltages between 2.0 and 5.8 kV. The sputtering system was basically a d.c. diode system but an electron-emitting filament was also used to increase the range of sputtering conditions and to control the current independently of the pressure. At pressures below 15 mtorr, with voltages between 2.0 and 5.8 kV, the film resistivity was 210 μΩ cm, but it increased with increasing pressure and decreasing voltage; at 50 mtorr, the resistivities of films sputtered at 4.0 and 2.0 kV were 270 and 2100 μΩ cm respectively. Resistivities as high as 8000 μΩ cm were obtained at 2.0 kV and 80 mtorr. As the resistivity increases from 200 to 8000 μΩ cm, there is a decrease in the film density from 14.5 to 8.2 g cm −3, the temperature derivative of resistivity becomes more negative and the β-Ta structure loses its strong (100) orientation. These effects are very similar to those reported for oxygen-doped tantalum films and the structure of the present films is probably similar. In this case, the oxygen is incorporated in the film both during and after sputtering. After deposition, measurements of the film resistance were made as a function of time. The resistivity increased when oxygen was supplied to the system and when the system was vented to atmosphere. The change in resistance increased with the final resistivity; increases between zero and 140% were recorded for films with final resistivities between 210 and 8000 μΩ cm respectively. The surfaces of the films, after reaching their final resistivities, were examined by electron microscopy. As the resistivity increased to 1100 μΩ cm, the film surface became increasingly rough due to the presence of large numbers of almost hemispherical hillocks with diameters of approximately 200 Å. With further increase in resistivity, the surface became smoother again.

Preparation and Corrosion Properties of a Tantalum Sputtered Thick Film

A tantalum sputtered film formed on the surface of stainless steel (AISI-316) has good corrosion resistance to hot sulphuric acid. Corrosion resistance of sputtered tantalum films, 6–20 μ thick is superior to that of other high quality corrosion resistant materials, such as niobium, titanium, zirconium, Hastelloy-B, and AISI-316 stainless steel. The corrosion rate of sputtered tantalum films is similar to that of 50 μ thick tantalum foil. Results of up to 1000 h corrosions tests, an adhesion strength test, and a bend test of the film are presented in this paper.

A new structure of sputtered tantalum

Electron microscopy at 100, 200 and 650 kV and X-ray diffraction analyses have been performed on sputtered tantalum films on thermally oxidized, silicon single-crystal substrates. The structure of both r.f. and d.c. sputtered films with no bias is body-centered cubic (b.c.c.) with a lattice parameter of 3.33 Å. However, the structure of the negatively biased d.c. sputtered films has been interpreted as a b.c.c. based superlattice structure with slight tetragonal distortion. The experimental values of the lattice parameters are a = b = 10.29 A ̊ and c = 9.2 A ̊ , with a c a ratio of 0.894. These results will be interpreted in terms of interstitial ordering and biaxial stresses in b.c.c. thin films. This interpretation of ordering of impurities in a b.c.c. lattice can resolve some of the inconsistencies which exist in the literature regarding the structure of sputtered tantalum.